Wetlands are a very challenging and stressful environment for both plants
and animals. The year-round presence of standing water, and the resulting
anaerobic conditions in the soil, require special adaptations for survival.
The salt marsh is an especially stressful and difficult wetland habitat.
In addition to the lack of oxygen in waterlogged soils, plants in the salt
marsh have to cope with high levels of salinity. Despite having to overcome
these two very challenging environmental conditions, salt marshes are among
the most productive habitats anywhere in the world. A typical Louisiana
salt marsh near Delacroix , La., southeast of New Orleans, had an NPP =
7,291 g/m-2/yr-1 (White et al.)

Salt marshes are widely distributed over most of the earth at middle
and higher latitudes. In tropical areas, salt marshes are replaced by mangrove
swamps. The rule of thumb used to distinguish between swamps and marshes
is that swamps are dominated by trees, which are generally lacking in marshes.

In North America, salt marshes occur all along the coast, especially
along the Gulf Coast. The entire coast of Louisiana is occupied by extensive
salt marshes, which grade into brackish marshes, then into freshwater marsh
in the upper Mississippi River delta. These coastal marshes depend on the
input of sediment from rivers. Because of the size and volume of the Mississippi
River, and its deltaic fresh and saltwater marshes, the Gulf Coast contains
about 60% of the coastal marsh land of the entire United States. These
coastal marshes are formed from a constant struggle between two opposing
forces - subsidence or lowering of the sea floor in the Gulf of Mexico,
and the raising of the marsh above sea level by the accumulation of organic
peat and the deposition of new sediment.

In the past, the Mississippi River was free to go where it wanted; it
wandered all over the map, changing its course entirely from time to time
(read Mark Twain’s autobiographical Life on the Mississippi). In colonial
times, when the Mississippi neared the Gulf it widened and slowed, forming
a series of wide loops or meanders. As it slowed down, it began to drop
the great load of sediment accumulated during its long journey. These sediments
were deposited in the shallow offshore waters of the Gulf. Much of that
sediment was trapped by the coastal marshes. After building up a vast delta,
the River would eventually find a new channel, and start to build up yet
another delta.

But now the river has been tamed and channeled between narrow banks
by the Army Corps. When you force a fluid into a narrower channel, you
increase its flow rate. Water shoots out the mouth of the River, and all
that fertile topsoil from the midwest now goes hurtling out over the edge
of the continental shelf. Very little is retained in the coastal marshes.
And the delta, deprived of these sediments, is being steadily eroded.

Coastal marshes have also been severely damaged by extensive canals
cut by the oil industry over the past several decades, during exploration
and drilling for oil and natural gas. These channels gradually widen through
wave action, destabilizing and gradually destroying huge tracts of marsh.
Salt marshes are being lost at an alarming rate. Southern Louisiana is
literally being nibbled to death by the Gulf. Forty square miles per year
of Louisiana vanishes into the Gulf every year. At that rate, within 100
years all of Terrebonne Parish will have disappeared beneath the waves.
New Orleans will be well on its way to being a seaport, probably another
Venice!

The coastal marshes now stand as our only buffer against the tremendous
forces of wind and water from the Gulf of Mexico. They are our best protection
from hurricanes. The Army Corps estimates that every 2.7 mi. healthy
marsh will dissipate one foot of tidal surge. The marshes are vital to
Louisiana, not just as a buffer for tropical storms, but also for their
bountiful seafood. Many commercial species live in these marshes, and the
marshes are a vast nursery for juvenile shrimp, crabs, redfish, sea trout,
and dozens of other commercially important species. Above and beyond their
economic value, they are one of the most biologically productive and interesting
ecosystems on the planet.

Permanently waterlogged soils are relatively poor in oxygen, which makesmarshes
a very difficult place for plants to grow in. Normal soils are very porous,
and these pores are usually filled with oxygen. But in waterlogged soils,
water replaces the air in these pores, and the soils become anaerobic.
This drastically changes soil chemistry, shifting the balance from an oxidizing
environment to a reducing environment, which creates toxic sulfides and
ferrous iron in the soil. Anaerobic soil conditions also slows decomposition
rates. But you don't find the rapid buildup of detritus that you observe
in the freshwater swamp, because the tide carries large quantities of detritus
out of the system. Salt marsh ecology is dominated by the twice daily tidal
flow. When the tide comes in, it brings a fresh supply of nutrients; when
the tide goes out, it carries off dead and decaying organic matter. About
45% of the NPP of the salt marsh is carried off into estuaries by the tidal
flow.

Salt marsh plants cope with anaerobic conditions in many different ways.
They are tolerant of high levels of toxic compounds formed in this reducing
environment. They preserve normal root uptake of water and nutrients, and
normal root respiration, by pumping oxygen into and out of the roots. This
creates a thin zone of oxygenated soil surrounding the roots (the same
as the tupelo trees in freshwater swamps). In addition, salt marsh plants
have a lot of aerenchyma tissue, tissues with ample air spaces. Up to about
60% of the plant bodymay be composed of aerenchyma. In the most stressful,
highly anaerobic parts of the marsh, plants also rely heavily on anaerobic
fermentation in their own metabolism.

But in addition to a lack of oxygen, plants also have to contend with
very high levels of salt in the water and in the soil. The physiological
effects of salt are similar to those of extremely arid conditions. Ironically,
these plants stand in water all day, but have some of the same adaptations
as desert plants (xerophytes), such as narrow leaves and sunken stomata.
Both of these adaptations reduce water loss. Many salt marsh plants can
also exclude salt from the roots while still taking up the water they need.
Some species (like certain mangroves), can exude salt from their leaves.
Marsh plants must pay a very high energetic price for growing in saline,
waterlogged soils. About 80% of the carbon fixed by salt marsh plants goes
into maintenance.

Very few species can survive the ecological challenge of too much salt
and too little oxygen. Consequently, even though salt marsh productivity
is very high, species diversity is very low. Salt marsh communities are
dominated by a few species of halophytes, plants that are adapted for growth
and reproduction in a saline environment. Louisiana salt marshes are dominated
by two species of Spartina. In brackish marshes, Spartina patens is the
dominant form, locally called cordgrass, wiregrass, marsh hay, or paille
a chat tigre (hair of the tiger). In saline marshes, Spartina alterniflora
(smooth cordgrass) dominates the net primary productivity of the marsh.
S. alterniflora comes in two ecotypes, a tall form that grows in the deeper
parts of the marsh, and a short form that grows in more elevated areas.
An ecotype is a variant phenotype of the same species that is adapted to
local conditions. Ecotypes like Spartina's tall and short forms seem to
be due to a combination of genetic and environmental influences. It's very
hard for seedlings to establish and grow in shifting sand and mud, so Spartina,
like many salt marsh plants, grows and spreads by means of rhizomes (horizontal
stems).

Salt marshes show a distinct zonation due to the effect of slight differences
in salinity and levels of inundation. Because marshes show a strong environmental
gradient, you find different species in the "high" marsh and the "low"
marsh. Species that share a similar range of tolerance to salt and water
levels grow together. So the salt marsh community shows an open community
structure, with species distributed at random with respect to one another,
according to their own particular range of environmental tolerance of salt
and standing water.

The flow of energy in the marsh is dominated by the NPP of Spartina,
with a small contribution from algae (about 1620 g/m-2/yr-1). The grasshopper
Orchelimum, and the sap-sucking leafhopper Prokelisia graze on Spartina,
along with a few other species. These herbivores are eaten in turn by other
arthropods, mainly spiders and insects. There are 81 species of spiders
and insects commonly found in southern salt marshes. But only about 4.6%
of Spartina NPP goes into the dominant herbivores.

Over 95% of the NPP goes into an entirely different energetic pathway.
Most of the plant biomass dies and decays, and most of the salt marsh energy
flow passes through the detritivores in the community. The primary consumers
in this detritivore food chain are bacteria and fungi. These are fed upon
by protozoa, nematodes, rotifers, larval invertebrates of several species,
and other predators. And these small creatures feed larger predators, like
polychaete worms, snails, bivalves, shrimp, and crabs. Some of these carnivores
also feed directly on the algae. There can be literally hundreds of crabs
and snails per square meter in a healthy salt marsh.

There are relatively few vertebrate animals in the salt marsh. A few
species of fish, like mullet, live there, and many species use the marsh
as a refuge for their young. A few mammals, like muskrat and nutria, can
survive in the marsh, as can about 10 species of reptiles and amphibians,
including the American Alligator. The most common vertebrates in the salt
marsh are birds. Only a few species of birds live exclusively in the salt
marsh, birds like the clapper rail, the seaside sparrow, and the long-billed
marsh wren. But many other birds feed in the marsh, including herons, egrets,
wood storks, spoonbills, and ducks.

Despite this abundance of visible life, in the salt marsh it is the
invisible creatures, the microscopic detritivores like bacteria and fungi,
that are the dominant channel of energy flow. About 47% of Spartina NPP
is ultimately lost in the respiration of the bacteria that feed on it.
The salt marsh food chain is a simple one -- Spartina --> Microbes
--> Animals.

Salt marsh communities, therefore, are characterized by:

1) Very high productivity
2) Very low species diversity
3) A food chain dominated by detritivores

As with the freshwater swamp, severe weather is the greatest source
of natural disturbance in salt marshes, but the effects of weather are
dwarfed by the disturbance caused by mankind. We have starved the marshes
for sediments by channeling the river, and further weakened it by carving
it into a spider web of canals. The ultimate demise of this ecosystem,
however, may be due to global warming. Some computer models of global warming
predict as much as a one meter rise in sea level over the next 100 years.
A one-meter rise in sea level may not sound especially drastic, but it
would completely wipe out 82% of our remaining coastal salt marshes.